This invention relates to marine seismic methods and more particularly to acoustic sources for use in offshore seismic exploration.
As distinguished from the "explosive" acoustic impulse sources such as air guns being known for a long time, vibration sources or transducers, in particular so-called flextensional transducers have been employed in seismic exploration in recent years. The invention is based on the utilization of such vibration transducers, i.e. flextensional transducers or other types of vibrational devices, including transducers of the piston type.
Examples of known flextensional transducers are found, inter alia, in Norwegian Patent No. 162,134 and U.S. Pat. No. 5,005,665. Both these known devices are somewhat special and directed to onshore seismic exploration. Known examples of acoustic source arrays or subarrays for seismic purposes, including among other things also vibration units or transducers, are found in U.S. Pat. Nos. 4,914,636, 4,918,668 and 5,281,773. None of these however, are particularly related to enhancement of the low frequency power of the acoustic-source.
Moreover, reference is made to an article in Geophysical Prospecting, 1980, 28, 323-332, by J. E. Sinclair and G. Bhattacharya: "Interaction effects in marine seismic source arrays". What is described therein are effects being obtained when acoustic sources in an array have a relatively large mutual spacing, ie. being outside the near field of each other.
A problem in connection with vibration transducers is to obtain a desired high acoustic power output at low frequencies, such as in the range down towards 100 Hertz. An improved low frequency response can be obtained by increasing the dimensions of the transducer units, but this involves practical and cost drawbacks.